Flashlamps are often used as illumination sources for machine vision or surveillance cameras. However, the flashlamp emits light in all directions, so a collimating optical system must be used to concentrate the light onto the target of interest. Cylindrical parabolic or paraboloid reflectors are often used to collimate the light from linear flashlamps, sometimes in conjunction with a fresnel refractive lens. Unfortunately these standard methods have a number of drawbacks.
Cylindrical reflectors can only collimate the light in one plane and have a very wide beam in a plane orthogonal thereto, so most of the light generated by the lamp is wasted. Adding a cylindrical fresnel lens helps, but the lens must have a long focal length and large aperture which makes it heavy and expensive. Refractive elements can also separate the colours of the light due to dispersion, which is often undesirable in applications involving optical sensors.
Paraboloid reflectors may capture most of the light, but in order to function well the illumination source must fit entirely within the paraboloid. For long flashlamps, this imposes a large diameter for the reflector, making the reflector large and expensive. Examples of this configuration are disclosed in U.S. Pat. Nos. 5,037,191 and 5,235,470, which generally describe paraboloid reflectors for use with a linear light source. Further, some configurations of paraboloid reflectors such as the ones found in U.S. Pat. Nos. 4,210,954 and 3,254,342 may produce a doughnut beam pattern which is often undesirable for imaging applications.
U.S. Pat. No. 3,938,162 describes an antenna system for single or plural beams providing continuously variable beamwidth selectively in one or both of two orthogonal senses. The system includes two parabolic cylindrical reflectors, which are positioned with the focal axes thereof orthogonally. A point of multibeam feed is mounted adjacent the main reflector on the focal axis of the sub-reflector in the Airy disc of the system. Simultaneous operation of telescoping sections of the two reflectors provides bidirectional zooming of the beam.
U.S. Pat. No. 4,208,661 describes an antenna system with two parabolic cylindrical reflectors with a point feed source located on the focal axis of one of the reflectors, wherein the directrix of the cylindrical wave front obtained by exposure of the first reflector coincides with the focal line of the second reflector, and the focal line of the first reflector is not parallel to the symmetry plane of the second reflector.
Therefore, it would be very advantageous to provide an improved reflector system for linear lamp illuminators.